The invention relates to a method for TIG welding.
TIG arc welding with alternating current is used to a great extent for the welding of metals such as aluminium and aluminium alloys. The polarity of current and voltage changes traditionally with 50 or 60 Hz synchronously with the frequency of the power supply to which the welding equipment is connected. Today, wide use is made of inverter-based current sources, and such a welding current source is a prerequisite for the present invention, in that it is hereby possible to adjust the welding machine for frequencies other than those mentioned of the alternating current which is used to transfer the energy to the welding arc.
In the welding process, use is made of the positive half cycle, i.e. when the welding electrode voltage is positive in relation to the workpiece, for cleaning the workpiece by removing the aluminium-oxide layer on the surface of the workpiece, and the negative half cycle is used to heat the workpiece so that a "weldpool" is formed. During the positive half cycle, approx. 1/3 of the power is dissipated in the workpiece and approx. 2/3 of the power is dissipated in the tunsten electrode. During the negative half cycle, approx. 2/3 of the power is dissipated in the workpiece and approx. 1/3 of the power is dissipated in the tungsten electrode.
When the polarity is changed, the arc must be re-ignited. This occurs easily when the change is from positive to negative polarity, in that the electrode is hot and easily emits electrons, whereas it is difficult to change from the negative polarity to positive polarity. This is partly due to the fact that the electrode is hotter than the weldpool, but also because pure aluminium does not easily emit electrons. The energy required for the emission of electrons from aluminium is very high, and since aluminium oxide is electrically insulating, such a current source must necessarily be equipped with a circuit which imparts a brief pulse with high voltage to the welding circuit in order to re-establish the positive half cycle. Moreover, it is necessary to emit electrons from the aluminium oxide which is constantly formed on the aluminium surface of the weldpool. This means that the positive half cycle can be established only if there are aluminium oxides within a certain radius from the tungsten electrode which is used to transfer the current to the workpiece. It is also necessary for the weldpool to be completely cleansed of aluminium oxide in order to achieve a satisfactory weld quality. In order to be able to adjust the cleansing effect so that it is appropriate for the relevant welding task, many welding current sources are equipped with a regulating element whereby it is possible to adjust the relationship between the time for which the polarity of the tungsten electrode is positive and the time for which the polarity is negative. Such a regulator element enables the cleansing effect to be regulated.
A welding apparatus which uses the known technique explained above is known from U.S. Pat. No. 4,947,021.
From U.S. Pat. No. 4,963,715 there is known another welding apparatus for TIG welding which is provided with a circuit which can detect whether or not the arc is re-ignited in the positive cycle. If the arc is not re-ignited, the ignition voltage is immediately increased to ensure that the arc is re-established.